The tolerance to global cardiac ischemia and reperfusion, necessary for repair of congenital defects in the young has been found to be poor in cyanotic patients compared to those without cyanosis. The purpose of these studies was to develop a colony of cyanotic young dogs to determine the effect of chronic hypoxenia on myocardial function and develop methods to improve tolerance to global ischemia. Foxhound puppies underwent the greation of a right to left shunt between pulmonary artery and left atrium using absorbable sutures. This created a level of cyanosis which was found to remain constant despite a 3 to 4 fold increase of weight over a 6 month adaptive period. Currently, we are involved in the second phase of this study. This involves acute of these animals at the end of this 6 month period and measurement of left ventricular function, high energy phosphates and myocardial metabolism via lactate flux and MV02. These measurements are done both before and after a period of cross clamp induces ischemia with correction of the shunt. Studies are incomplete in the above measurements both before and after operation. In this way we hope to gain some insight into the mechanisms responsible for the reported higher morbidity and mortality often reported in children with cyanotic congenital heart disease.